Electrical sheets, silicon steel sheets and the like have heretofore been widely used as soft magnetic parts such as cores in electric apparatus such as electric motors as is well known in the art. Recently, sintered magnetic materials formed by compacting and sintering iron powder have progressively replaced the electrical sheets and silicon steel sheets. These sintered materials have some advantages characteristic of powder metallurgy including an increased percent yield based on the stock, a low processing cost, and an increased degree of freedom in shape, but have the disadvantage that their magnetic characteristics are imperatively inferior to those of electrical sheets and silicon steel sheets due to residual voids in the sintered materials.
To overcome the drawbacks of sintered iron base materials as mentioned above, attempts have been made to add a variety of additives. Among such additives, tin (Sn) forms a liquid phase at a relatively low temperature. If tin is added, a liquid phase is created during sintering and tin forms a solid solution with iron to allow .alpha.-phase iron to develop during sintering, resulting in an increased sinter density, reduced influence of voids, and promoted growth of .alpha.-phase crystals, and hence, the possibility of achieving excellent magnetic characteristics. If high density sinters are made by adding tin, then it is expectable to apply them to sintered mechanical parts requiring wear resistance and high strength.
Known among processes for adding tin to sintered iron-base material is a process comprising mixing a tin powder with an iron powder, compacting the mixture and sintering it, as disclosed in Japanese Patent Application Kokai No. 48-102008. In this process, however, since tin is melted during sintering to penetrate between iron particulates to spread the interstices between them and depleted voids are left where tin particulates have occupied before melting, the sinter density is not sufficiently increased in practice, failing to provide satisfactory magnetic characteristics.
In order to overcome such problems, it may be contemplated to use a powder iron alloy which has previously been alloyed. However, alloying with tin makes iron base powder harder to considerably deteriorate its compressibility to provide a reduced compact density although the development of tin-depleted voids is prevented. It is thus eventually difficult to provide a high sinter density.
It may also be contemplated that if very finely divided tin is used as the metallic tin powder to be added to and blended with an iron powder, then depleted voids of a substantial size are not left even after melting of tin during sintering so that uniform sintering may take place to yield a coherent sinter. However( atomizing and triturating techniques normally employed in the preparation of tin powder are difficult to effectively produce such very finely divided tin.
It is, therefore, a primary object of the present invention to provide an Sn-containing iron base sintering powder stock which can be converted into a coherent sinter having improved magnetic characteristics.
It is a secondary object to provide a process for efficiently making an Sn-containing iron base sintering powder stock in an industrial scale, the powder stock being convertible into a coherent sinter having improved magnetic characteristics.
It is a further object of the present invention to provide an Sn-containing iron base sintering powder stock which may be converted into Sn-containing iron base sinters exhibiting high strength and high wear resistance in their applications other than as megnetic parts, for example, application as mechanical parts or the like, as well as a process for making such a powder stock.